1. Field of the Invention
This invention relates to signal propagation and, more particularly, to novel and highly effective signal propagation using a dual-probe waveguide cavity.
2. Description of the Prior Art
Because of the crowding of the electromagnetic spectrum it is the practice to use polarized signals for transmissions between, for example, satellites and ground stations. A typical case is the use of electromagnetic signals having electric fields that are linearly polarized respectively vertically and horizontally, or electric fields that are elliptically (preferably circularly) polarized respectively clockwise and counterclockwise.
Antennas are bidirectional devices. That is, they can be used for transmitting or receiving electromagnetic signals. An antenna that selectively receives signals of a given polarity can be used to selectively transmit signals of the same polarity. For the sake of concreteness, however, the following introduction is in terms of an antenna employed to receive signals.
A probe mounted in a waveguide cavity receives energy from an incoming polarized signal having an electric field in the plane of the probe and is essentially invisible to an incoming signal having an electric field normal to the plane of the probe. In order to receive signals that are linearly polarized with the electric field oriented vertically, a waveguide cavity is provided with a probe lying in a vertical plane; similarly, in order to receive signals that are linearly polarized with the electric field oriented horizontally, a waveguide cavity is provided with a probe lying in a horizontal plane. Incoming circularly and elliptically polarized signals are in effect converted within the waveguide cavity to signals that are polarized linearly. This can be done in a well known manner using a dielectric plate, for example.
Ideally, the probe is centered radially within the waveguide cavity because the electric field curves within the cavity, as explained below, and is normal to the walls defining the cavity. The best cross-polarization nulls between orthogonally polarized electric fields are obtainable at the radial center of the cavity, where the vertical and horizontal electric fields are truly orthogonal. Elsewhere, they form varying angles with each other.
To receive both signals that have linearly polarized vertical electric fields and signals that have linearly polarized horizontal electric fields, several strategies can be adopted: Obviously, separate waveguide cavities can be provided, one including a probe lying in a vertical plane and the other including a probe lying in a horizontal plane. Alternatively, a single waveguide cavity can be provided with a single centrally positioned probe and means for changing the orientation of the end of the probe nearer the open end of the cavity while maintaining the rear end of the probe fixed. The front end of the probe then can pick up either signal and reorient the electric vector to the extent necessary for reception and transmission at the rear end. Of course, only signals of one polarization can be received at any given time using this technique. A third possibility is to employ multiple fixed probes in the same cavity.
FIG. 1 is a stylized representation of vertically polarized electric field lines E within a waveguide cavity in the TE.sub.11 transmission mode. The number of field lines E represented in FIG. 1 is of course completely arbitrary. One of these field lines, designated E1, bisects the cavity and is straight (the cavity is assumed to be cylindrical; i.e., circular in cross section). The other field lines E are curved, and each is normal to the wall of the cavity, which is electrically conductive. If FIG. 1 is rotated through 90.degree. clockwise or counterclockwise, it represents horizontally polarized electric field lines within the waveguide.
A single conductive probe mounted in the center of the cavity can easily be made to respond to the signal by orienting it so that it lies in a plane parallel to the central electric field line El. By the same token, it can easily be made invisible to the signal by orienting it so that it lies in a plane perpendicular to the electric field line E1.
If two probes are mounted in the same cavity to receive differently polarized signals, obviously both cannot occupy the central position; they must occupy separate positions. This creates a problem of optimizing the responsiveness of each probe to polarized signals.
FIGS. 2-7 show prior attempts to address the problem. FIGS. 2 and 3 show a design that is disclosed in more detail in a patent to West U.S. Pat. No. 5,216,432. In this design, probes 10 and 12 enter the cavity 14 from the bottom 16 of the circular (actually cylindrical) waveguide 18 and have structures 20, 22 protruding from the walls of the waveguide 18 that set the impedance of the probes 10, 12 coming from the waveguide floor 16. In addition, there is an impedance-matching structure 24 in the center of the waveguide 18. The combination of these two provides adequate cross polarization and return loss from the probe to the waveguide. The additional structures 20, 22, 24 are necessary to decrease the cross talk between the two probes 10, 12 and increase the cross polarization performance by altering the field at the base of the probes 10, 12. However, these structures have the disadvantage of introducing complexity. Note also that the probes 10 and 12 lie in planes that are at 90.degree. relative to each other, corresponding to the relative orientation of the central, bisecting electric fields within the waveguide 18.
FIGS. 4 and 5 disclose a waveguide 26 that is lengthened and has two wire probes 28, 30 that enter the waveguide 26 from the side at right angles to each other. A septum 32 divides the waveguide 26 from the standpoint of the two polarities. A disadvantage of this design is that the probes enter the printed circuit board 34 at angles of 45.degree., the waveguide is lengthened, and a septum is required to minimize cross talk between the probes 28, 30 and maintain high cross-polarization isolation.
FIGS. 6 and 7 disclose a waveguide 34 having a pair of probes 36, 38 that enter the waveguide from the bottom and lie in planes that form right angles with respect to each other. In this design, cross talk between the two probes and the structure of the fields in the waveguide are such that the cross-polarization isolation is reduced.
Thus in the prior art, additional structures are required in order to achieve the desired performance, or the achieved performance is less than optimum.